Field of the Invention
The invention relates to a catalyst body for breaking down nitrogen oxides in the presence of a reducing agent. The catalyst body has an active material that contains a zeolite and titanium dioxide. The invention also relates to a process for breaking down nitrogen oxides in a gas stream, a gas stream that contains nitrogen oxides being passed over the catalyst body. In particular, at the catalyst body the nitrogen oxides, with the aid of the reducing agent and in the presence of oxygen, are converted into molecular nitrogen and water using a selective catalytic reduction (SRC) process.
A catalyst body of the type described in the introduction is known from published British Patent Application GB 2 193 655 A. The active material of the catalyst body described in that document contains a titanium dioxide with a small specific surface area and a zeolite that is obtained by ion exchange and contains copper. The zeolite has a mean pore diameter of 10 xc3x85 or less and a molar ratio of silicon oxide to aluminum oxide of 10 or more. The catalyst body described has a high mechanical strength and a good resistance, in terms of its catalytic activity, to volatile catalyst poisons, such as arsenic, selenium or tellurium. Mordenite, ZSM-5 and ferrierite are described as preferred zeolites.
Furthermore, Published, European Patent Application EP 0 393 917 A2 discloses a catalyst body for breaking down nitrogen oxides, the active material of which contains a zeolite which, after ion exchange, contains copper and/or iron. The zeolite has a molar ratio of silicon oxide to aluminum oxide of at least 10 and a pore structure in which channels in all three spatial directions have a diameter of at least 7 xc3x85. The catalyst body is supposed to be suitable for breaking down the nitrogen oxides in a temperature range from 250 to 600xc2x0 C. USY (Ultra-Stabilized Y), Beta and ZSM-20 are described as preferred zeolites.
By contrast, conventional catalyst bodies with an active material which contains titanium dioxide and additions of vanadium oxide, tungsten oxide and/or molybdenum oxide are only suitable for breaking down nitrogen oxides up to a temperature of approximately 450xc2x0 C. Since exhaust gases from a combustion installation, such as for example a fossil-fired power plant, a gas turbine or an internal-combustion engine, which contain nitrogen oxides regularly reach temperatures of up to 500xc2x0 C. and above, the catalyst body described in Published, European Patent Application EP 0 393 917 A2 offers a considerable advantage.
U.S. Pat. No. 5,271,913 discloses a catalyst body for breaking down nitrogen oxides, the active material of which body contains a zeolite. The zeolite is in this case impregnated with cerium oxide or an iron oxide. The catalyst body is suitable for breaking down the nitrogen oxides using the selective catalytic reduction process in a temperature range from 500 to 700xc2x0 C. Furthermore, the catalyst body described has a high stability with regard to sulfur components contained in the exhaust gas. A zeolite of the ZSM-5 type is described as a preferred zeolite, the molar ratio of silicon oxide to aluminum oxide being 20 or more.
It is accordingly an object of the invention to provide a catalyst body and a process for breaking down nitrogen oxides that overcome the above-mentioned disadvantages of the prior art devices and methods of this general type, which is still suitable for breaking down nitrogen oxides in the presence of a reducing agent even in a temperature range from 400 to 750xc2x0 C. For this purpose, the catalyst body is to have both a sufficient mechanical stability and a sufficient catalytic stability. A further object of the invention is to describe a process for breaking down nitrogen oxides in a gas stream, with which it is possible to effectively lower the level of nitrogen oxides even at gas temperatures of between 400 and 750xc2x0 C.
With the foregoing and other objects in view there is provided, in accordance with the invention, a catalyst body for breaking down nitrogen oxide in a presence of a reducing agent. The catalyst body contains an active material having a hydrogen-ion-exchanged, acid zeolite and an active component. The active material contains 40-60% by weight of the zeolite and 40-60% by weight of the active component. The active component contains 70-95% by weight of titanium dioxide, 2-30% by weight of tungsten trioxide, 0.1-10% by weight of aluminum oxide and 0.1-10% by weight of silicon dioxide.
The first object is achieved by a catalyst body having an active material that contains a zeolite and titanium dioxide, according to the invention, by the fact that the zeolite is a hydrogen-ion-exchanged, acid zeolite.
The term hydrogen-ion-exchanged, acid zeolite is understood as being a zeolite in which the exchangeable cations have been predominantly exchanged for hydrogen ions. This can take place, for example, by thermal conversion of ammonium (NH4+) ions which are contained in synthetic zeolites, by hydrogen ion exchange or by hydrolysis of a multiply charged cation-containing zeolite during a dehydration. In this context, reference is made in particular to Kirk-Othmer, xe2x80x9cEncyclopedia of Chemical Technologyxe2x80x9d, 3rd Edition, Volume 15, John Wiley and Sons, New York, 1981, pages 640 to 669.
Unlike in the prior art, it is not necessary, in the catalyst body according to the invention, for the zeolite of the active material to be a metal-cation-exchanged, i.e. for the exchangeable cations of the zeolite to be exchanged for metal cations, for example of copper or iron.
It should be noted that the term zeolite is understood as meaning a framework aluminosilicate in which the ratio of the oxygen atoms to the sum of the aluminum and silicon atoms is 2:1. As a result of some silicon atoms of oxidation state IV being exchanged for aluminum atoms of oxidation state III, the framework or the framework structure overall acquires a negative charge. The negative charge is compensated for by cations that are in the framework structure. The cations are what are known as exchangeable cations that can readily be replaced by other cations, in particular metal cations, by ion exchange.
A zeolite is also distinguished by the fact that the framework structure has continuous pores with a characteristic pore width.
Zeolites are classified on the basis of the molar ratio of silicon oxide to aluminum oxide or according to the characteristic framework structure resulting from the molar ratio. For classification purposes, reference is made to the article xe2x80x9cChemical Nomenclature and Formulation of Compositions of Synthetic and Natural Zeolitesxe2x80x9d by R. M. Barrer, Pure Appl. Chem. 51 (1979), pages 1091 to 1100.
An example of a natural zeolite is mordenite or a chabazite. Examples of synthetic zeolites are A, X and Y zeolites, which represent synthetic forms of mordenite, a ZSM-5 zeolite (ZSM-5 being a trademark for a synthetic zeolite produced by Mobil Oil Company Ltd.), an USY (Ultra-Stabilized Y) zeolite or a Beta zeolite. With regard to the structure of mordenite, of ZSM-5 zeolite and of Y zeolite, reference is also made to the specialist article titled xe2x80x9cAciditxc3xa4t der Lewis-Zentren in Zeolith-Katalysatorenxe2x80x94NO als Sondenmolekxc3xclxe2x80x9d [xe2x80x9cAcidity of the Lewis Centers in Zeolite Catalystsxe2x80x94NO as Probe Molecule], by Frank O. Witzel, Fortschrittberichte VDI, Series 3: Verfahrenstechnik, No. 292, 1992.
Extensive tests have shown that a catalyst body with an active material which contains titanium dioxide and a hydrogen-ion-exchanged, acid zeolite is suitable for catalytic reduction of the nitrogen oxides using the SCR process up to temperatures of 750xc2x0 C. This is because a catalyst body of this type on the one hand is catalytically active up to these high temperatures and on the other hand has the requisite temperature stability. In addition, the catalyst body has a high degree of stability with respect to moisture and a high resistance to sulfur-containing components in an exhaust gas that is to be treated.
The catalyst body opens up the possibility of reducing nitrogen oxides in exhaust gases from an internal-combustion engine or a gas turbine, it being possible for very high temperatures of the exhaust gas to occur without additional measures having to be taken in order to reduce the temperature so as to protect the catalyst body.
In a preferred embodiment, the active material of the catalyst body contains 40 to 60% by weight of zeolite. This composition provides a particularly good temperature stability and particularly low deactivation of the catalytic activity at high temperatures.
Furthermore, it is advantageous if the active material contains 40 to 60% by weight of an active component which contains, in each case based on the weight of the active component, 70 to 95% by weight of titanium dioxide, 2 to 30% by weight of tungsten trioxide, 0.1 to 10% by weight of aluminum oxide and 0.1 to 10% by weight of silicon dioxide. As a result, the catalyst body has a high catalytic activity with regard to the reduction of nitrogen oxides using the SCR process, i.e. for breaking down nitrogen oxides in the presence of a reducing agent.
It is particularly advantageous if the active component contains 8 to 12% by weight of tungsten trioxide.
Furthermore, it is advantageous if an USY zeolite, a Beta zeolite or a ZSM-5 zeolite is used as the zeolite. On account of its framework structure, a zeolite of this type is particularly suitable for the desired catalytic use.
For the catalytic activity, it is also advantageous if the active material has a BET surface area of 30 to 150 m2/g and a pore volume, measured using the Hg penetration method, of 100 to 1000 ml/g.
After the hydrogen-ion-exchanged, acid zeolite has been prepared, the active material of the catalyst body can be produced, in a manner known per se, as follows. A starting material is produced, including the zeolite, by mixing, milling and/or kneading the individual components or their precursor compounds (for example water-soluble salts for the specified metal oxides) and if appropriate with the addition of conventional ceramic fillers and auxiliaries and/or glass fibers. The starting material is then either processed further to form unsupported extrudates or is applied as a coating to a ceramic or metallic support in honeycomb or plate form. The starting material is then dried at a temperature of 20 to 100xc2x0 C. After the drying operation, the starting material is calcined to form the active material by calcination at temperatures of between 400 and 700xc2x0 C.
In addition, after the calcining process, the calcined active material can be artificially aged by a final heat treatment at a temperature that is higher than the calcining temperature. A temperature which is approximately 50xc2x0 C. above the subsequent maximum temperature of use of the catalyst body is selected for the artificial aging. The final heat treatment is carried out for a period of 20 to 80 hours.
In this way, the catalyst body acquires an improved temperature resistance.
The object relating to a process for breaking down nitrogen oxides in a gas stream is achieved, according to the invention, by the fact that a gas stream which contains nitrogen oxides is passed, in the presence of a reducing agent, over the described catalyst body, the nitrogen oxides being converted into nitrogen and water.
It is advantageous for the process and particularly cost-effective if ammonia or an aqueous urea solution is added to the gas stream as a reducing agent.
Advantageously, the gas stream is passed over the catalyst body at a temperature of 250 to 750xc2x0 C. Within this specified temperature range, effective conversion of the nitrogen oxides into nitrogen and water takes place. There is no likelihood of the active material of the catalyst body being deactivated.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a catalyst body and a process for breaking down nitrogen oxides, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.